A conventional ultrasound imaging system comprises an array of ultrasonic transducer elements which transmit a steered ultrasound beam and then receive the reflected beam from the object being studied. Scanning of the object comprises a series of measurements in which the steered ultrasonic wave is transmitted, the system switches to receive mode after a short time interval, and the reflected ultrasonic wave is received, beamformed and processed for display. Typically, transmission and reception are steered in the same direction during each measurement to acquire data from a series of points along an acoustic beam or scan line. The receiver is dynamically focused at a succession of ranges along the scan line as the reflected ultrasonic waves are received.
For ultrasound imaging, the array typically has a multiplicity of transducer elements arranged in one or more rows and driven with separate voltages so as to act as transmit elements. By selecting the time delay (or phase) and amplitude of the applied voltages, the individual transducer elements in a given row can be controlled to produce ultrasonic waves which combine to form a net ultrasonic wave that travels along a preferred vector direction and is focused at a selected point along the beam. The beamforming parameters of each of the firings may be varied to provide a change in maximum focus or otherwise change the content of the received data for each firing, e.g., by transmitting successive beams along the same scan line with the focal point of each beam being shifted relative to the focal point of the previous beam. By changing the time delay and complex amplitude of the applied voltages, the beam with its focal point can be moved in a plane to scan the object.
The same principles apply when the transducer probe is employed to receive the reflected sound in a receive mode. The voltages produced at the receiving transducer elements are summed so that the net signal at a particular time is indicative of the ultrasound reflected from a single focal point in the object. As with the transmission mode, this focused reception of the ultrasonic energy is achieved by imparting separate time delays (and/or phase shifts) and gains to the signal from each receiving transducer element. The output signals of the beamformer channels are then coherently summed to form a respective pixel intensity value for each sample volume in the object region or volume of interest. These pixel intensity values are log-compressed, scan-converted and then displayed as an image of the anatomy being scanned.
High frame-rate systems are desirable for present-day two- dimensional (2D) imaging and necessary for future real-time three- dimensional (3D) imaging. The frame rate of medical ultrasound imaging systems is determined by the number of transmit events necessary per frame. In conventional ultrasound imaging systems a transmit event is a focused beam transmitted in a particular direction or at a particular focal position.
Frame rate in medical ultrasound imaging is a valuable resource. With increased frame rate, larger regions (as in color flow or three-dimensional imaging) or faster objects (heart) can be imaged. Image enhancement methods such as video integration (noise reduction) or compounding (speckle reduction) can also use up frame rate.
In conventional medical ultrasound imaging, a single pulse is transmitted in a particular direction and the reflected echoes are coherently summed to form a single line in the image frame. The amount of time necessary to form such scan line is determined largely by the round-trip transit time of the ultrasonic pulse. Many scan lines are present in an image frame to densely sample the anatomical region of interest. Thus, the frame rate in conventional medical ultrasound imaging is determined by the sound propagation speed and the size of the region of interest.
The frame rate can be improved by decreasing the number of transmit events per frame. Conventionally, this has been accomplished with a proportional reduction in the number of transmit elements used in each transmit event, resulting in poor signal-to-noise ratio (SNR).
Synthetic transmit aperture imaging has the potential to increase the present frame rate by over an order of magnitude. A method for synthetic transmit aperture imaging at a 1 kHz frame rate was proposed by Lockwood et al. in "Design of Sparse Array Imaging Systems", 1995 IEEE Ultrasonic Symp. Proc., pp. 1237-1243. Standard synthetic transmit aperture imaging, as described by Lockwood et al., is based on transmitting consecutively from different point sources, each with broad area coverage, in order to obtain point-to-point data that is subsequently beamformed. A point source comprises a single element or a group of elements phased to form a cylindrical/spherical diverging wave. Unlike conventional imaging where each image point is formed using echoes from just one transmit event, synthetic transmit aperture imaging uses echoes from several consecutive transmit events (or "transmits") that are coherently combined to form each image point. The number of transmits per image frame that can be used is limited by tissue motion and is much smaller than the number used in conventional imaging. This small number of transmits results in a very high frame rate, but it also limits the image SNR because the number of point sources cannot exceed the number of transmits. A problem to be solved is how to improve the SNR when imaging with a predetermined number of transmits and transmit elements.